Integrated circuit (IC) fabrication commonly involves sequential deposition and etching steps to pattern the materials that comprise and integrate the discrete devices. Radiation sensitive materials (photoresists) are commonly used to transfer device patterns into functional materials. Processing steps include pattern-defined radiation exposure, pattern development, and then transfer into the functional material via differential etching. Because continued improvement in IC performance is dictated by reduction in device size, progress is gated by advances in photolithographic process resolution. The development of improved resolution photoresists with conventional organic materials, however, is highly constrained by their intrinsic chemical and optical properties. For this reason, various underlayer materials having appropriate optical and chemical properties are now widely used to enhance pattern transfer into functional materials.
As the feature sizes of integrated circuits continue to decrease, thinner layers of photoresist are used to prevent pattern collapse at high-aspect ratios. When combined with the limited etch selectivity between organic photoresists and functional materials, thin resist layers do not have the etch budget to transfer patterns directly into the functional device materials or the substrate. Hardmask (HM) underlayers have been widely adopted as a solution. The developed pattern from the thin photoresist is first etch transferred to the hardmask underlayer with an etch chemistry that has a much higher etch rate in the hardmask than in the resist. The pattern is then transferred from the hardmask into the functional layer with a different etch chemistry. In this case, the etch rate of the functional layer is much higher than that of the hardmask.